North Korea announced Tuesday it has conducted an underground nuclear test after seismic experts in the U.S. and other countries said they had detected an "artificial earthquake" in the reclusive nation.

Pyongyang's defiant action, its third atomic blast, drew global condemnation, with China summoning the North Korean envoy in Beijing to express its "strong dissatisfaction and firm opposition."

"A third nuclear test has been successfully staged," the state-run Korean Central News agency said in a dispatch after seismic activity measuring 4.9 magnitude was registered in North Korea by the U.S. Geological Survey.

KCNA said the test was conducted in a safe manner and "to protect our national security and sovereignty against the reckless hostility of the United States that violated our republic's right for a peaceful satellite launch."

Pyongyang warned even further provocations.

The test "was only the first response we took with maximum restraint," a spokesman for the North Korean Foreign Ministry said in a statement, KCNA reported.

"If the United States continues to come out with hostility and complicates the situation, we will be forced to take stronger, second and third responses in consecutive steps."

The nuclear test came a month after it successfully launched a long-range rocket in violation of U.N. resolutions that bans it from developing missile or nuclear technology.

Condemned

U.N. Secretary-General Ban Ki-moon condemned the test, saying it was a "clear and grave" violation of resolutions of the Security Council, which convened an emergency meeting Tuesday as world powers sought prompt action against North Korea.

U.S. President Barack Obama called the blast "a highly provocative act" that "undermines regional stability" and "increases the risk of proliferation."

"The danger posed by North Korea’s threatening activities warrants further swift and credible action by the international community," Obama said in a statement.

Russia and South Korea also slammed Pyongyang's action while Japanese Prime Minister Shinzo Abe said the test was "extremely regrettable."

A nuclear test monitoring agency in Vienna, Austria, had said earlier that the unusual seismic event in North Korea showed "clear explosion-like characteristics," indicating it occurred at about the same place as previous tests by North Korea.

Tibor Toth, executive secretary of the Comprehensive Nuclear Test Ban Treaty Organisation (CTBTO), said the seismic event's location was "roughly congruent with" nuclear tests carried out by North Korea in 2006 and 2009.

Nuclear blasts can create tremors but they are distinct from those caused by natural earthquakes.

The two previous tests were conducted at the Punggye-ri nuclear test site, in a remote mountainous region in the northeast, about 60 miles (100 kilometers) from the border with China and 120 miles (200 kilometers) from the Russian border.

Warnings defied

North Korea had announced plans for the latest nuclear test in response to sanctions imposed in January after the rocket launch, although satellite imagery indicated it has been readying its test site for more than a year.

The third test on Tuesday came despite strong warnings against any such blast by China, North Korea's top ally and aid provider.

Beijing urged its neighbor "to honor its commitment to denuclearization, and not to take any actions which might worsen the situation."

"What is clear is that North Korea has again defied the UN Security Council as well as renewed efforts by China to prevent its troublesome neighbor from ratcheting up tension on the Korean Peninsula," said Korea expert Bruce Klingner of the Washington-based Heritage Foundation.

Young North Korean leader Kim Jong Un, who took power following the death of his father, Kim Jong Il, in December 2011 "has shown himself to be no less belligerent or dangerous than his predecessors," he said.

Security Council to meet

The U.N. Security Council met in New York for emergency consultations.

The United States, South Korea and European members are expected to ask the council to impose new sanctions on Pyongyang.

"What the North Koreans have done now is a big challenge to the Chinese," a U.N. diplomat told Agence France-Presse. Any new sanctions against Pyongyang may not be agreed upon on Tuesday but the "intention" would be made clear, the envoy said.

South Korea, still technically at war with the North after the 1950-53 civil war ended in a truce, said the size of the seismic activity indicated a nuclear explosion slightly larger than the North's two previous tests at 6-7 kilotons.

The Hiroshima bomb was around 20 kilotons.

Reported by RFA's Korean Service. Written in English by Parameswaran Ponnudurai and Joshua Lipes.